In recent years, a PET bottle has been increasingly used in the field of beverage containers. An opening portion of the PET bottle is sealed by screwing a pilfer-proof cap made of a synthetic resin onto a threaded neck portion, and is resealable by screwing the cap again onto the threaded neck portion, even after opened by turning the cap to the left.
On the other hand, a can for a canned product or a beverage container competing with the PET bottle is numerously suggested by e.g., Japanese Utility Model Laid-Open No. 56-24431, Japanese Utility Model Laid-Open No. 61-51314, Japanese Patent Laid-Open No. 10-509095 (corresponding to the international publication No. WO96/15865, and corresponding to U.S. Pat. No. 5,718,352), Japanese Patent Laid-Open No. 2000-191006, WO01/15829 (corresponding to U.S. Pat. No. 6,499,329), WO01/23117 (corresponding to U.S. Pat. No. 6,463,776), Japanese Patent Laid-Open No. 2001-213417 and so on. The can comprises a diametrically small threaded cylindrical neck portion having a reseal function with a threaded cap, an inclining shaped shoulder portion and a diametrically large cylindrical trunk portion.
In WO01/15829, there is disclosed a type of a bottle-shaped can (i.e., a three-piece type) in which a diametrically small cylindrical threaded neck portion, a domed shoulder portion, and a diametrically large cylindrical trunk portion are integrally shaped from an aluminum alloy sheet, and in which a threaded cap (i.e., a pilfer-proof cap) made of an aluminum alloy sheet is mounted detachably on a neck portion of a can body wherein a bottom end (or a bottom wall) made of the aluminum alloy sheet is fixed an end portion of the trunk portion by double seaming method. In Japanese Patent Laid-Open No. 2001-213417, moreover, there is disclosed a type of the bottle-shaped can (i.e., a two-piece type) in which a threaded cap made of an aluminum alloy sheet is mounted detachably on the neck portion of a can body, the diametrically small cylindrical threaded neck portion, a frusto-conical shoulder portion, the diametrically large cylindrical trunk portion and a domed bottom portion of which are integrally shaped from the aluminum alloy sheet. In Japan, those bottle-shaped cans have been used for many kinds of beverages in recent years, such as a beer, a low-malt beer, a Japanese sake, a sparkling wine, a fruit juice, various types of carbonated beverages, a green tea, an oolong tea, a black tea, a coffee and so on.
As compared to the transparent PET bottle, the bottle-shaped can has excellent gas barrier performance and light intercepting effect. Therefore, the bottle-shaped can is excellent in, e.g., quality preservation to prevent the deterioration of quality of the beverage filled and sealed therein during storage or in stores. Similarly to the PET bottle, moreover, the bottle-shaped can may be resealed with the cap unless a content of the can e.g., the beverage is completely consumed. Furthermore, after the beverage is consumed, the bottle-shaped can be easily collected and recycled through existing an aluminum can recycling system. Thus, in view of recycling, the bottle-shaped can is superior to the PET bottle which does not have its own recycling system. For these reasons, the bottle-shaped can is expected to be used as the container for many more beverages. Since these advantages of the bottle-shaped can are obtainable also from the bottle-shaped can made mainly of a steel sheet (at least the can body is made of steel), the bottle-shaped can made of steel sheet is expected to be available.
In the bottle-shaped can of this kind, the diametrically small cylindrical neck portion, the domed or frusto-conical shoulder portion and the diametrically large cylindrical trunk portion are integrally shaped from the metal sheet, and both inner and outer faces of those portions are coated with the resin film. Moreover, the curled portion is formed annularly along an upper end opening edge of the neck portion where the thread is formed on its peripheral wall.
If an inwardly curled portion is formed on the opening portion (i.e., the upper end portion of the neck portion) of the bottle-shaped can, the curled portion obstructs a flow of the beverage so that it is hard for a consumer to let the beverage come out smoothly, when he opens the can and drinks the content, i.e., the beverage. Moreover, in case of resealing the can by the cap with the content remained therein, a hygiene situation gets worse due to the beverage adhered to the curled portion. Furthermore, since the trim end of the curled portion is situated inside of the can, the trim end of the curled portion (i.e., a trim end face where the metal sheet is exposed) gets corroded due to the beverage filled in the can, unless the trim end of the curled portion is especially be coated.
For these reasons, the outwardly curled portion is basically adopted to the metal can having the curled portion.
In case of forming the outwardly curled portion at the opening portion of the can, in addition, it is known in the prior art (e.g., Japanese Published Examined Application No. 56-14051, Japanese Utility Model Laid-Open No. 56-24431, Japanese Utility Model Laid-Open No. 61-51314, Japanese Utility Model Laid-Open No. 62-22945 etc.) that the curled portion is formed by curling and confining the trim end inward of the curled portion.
Here, the outwardly curled portion as formed at the opening portion of the aforementioned bottle-shaped can should have enough strength to withstand great pressure applied from above when the can is sealed with the cap after filling the beverage therein. Also, since the final products filled with the beverages and sealed (i.e., canned beverages) subject to drop impact when they are on transportation or in storage, or on display in store in a carton case, the opening curled portion should have sufficient deformation resistance against such a drop impact. Specifically, if the curled portion is deformed due to its insufficient deformation resistance (or strength), the sealability cannot be maintained in adequate manner between a seal member and the curled portion. Inadequate sealability causes a leakage of the beverage from between the neck portion and an inner face of the cap, and the wet carton case and contaminated other cans lead to mold growth in the carton case and the surface of the can.
On the other hand, the pilfer-proof cap, which is mounted on the neck portion of the bottle-shaped can made of an aluminum alloy sheet used as a positive pressure can (i.e., a can, in which an internal pressure is higher than an atmospheric pressure), generally has on its an upper portion a plurality of small holes called “vent slit” in a circumferential direction for the purpose of gas ventilation when opening the bottle-shaped can. If the vent slits are not provided, the cap may be blown off by a gas pressure generated between the cap and the neck portion when the can is opened. For this reason, the vent slits are so provided as to remove the gas pressure.
The can is hermetically sealed with a seal member attached to a rear face of a top plate of the cap. However, the vent slits opened between the neck portion and the cap allows moisture in the air or water to get therethrough even after the can is sealed with the cap. Such moisture or dew condensation thereof due to a cooling of the can or an abrupt change of the temperature of outside may corrode the trim end (i.e., a trim end face where the metal sheet is exposed) of the curled portion at the upper end of the neck portion.
The corrosion of the trim end of the curled portion caused by the moisture is not so serious problem for an aluminum can. However, in case of a steel can, the trim end of the curled portion gathers rust (i.e., red-rust) so that the commercial value of the can may be deteriorated significantly. In order to avoid such disadvantage, it is necessary to prevent the water from adhering to the trim end of the curled portion.
To this end, it is conceivable to apply a coating treatment to the trim end (i.e., a trim end face where the metal sheet is exposed) of the curled portion with a liquid coating compound or a fused thermoplastic resin. When applying such a coating treatment to the trim end of the curled portion, however, problems are often caused such as a scattering of the liquid coating compound or a stringing of the resin. In order to avoid these problems, it is necessary to develop a new apparatus and a new technology.
As has been described already, on the other hand, in case of forming the outwardly curled portion at the opening portion of the can, the curled portion is formed such that the trim end of the curled portion is rolled in and confined. This is known in the art by, e.g., Japanese Published Examined Application No. 56-14051, Japanese Utility Model Laid-Open No. 56-24431 (ref. FIG. 6), Japanese Utility Model Laid-Open No. 61-51314 (ref. FIG. 6), Japanese Utility Model Laid-Open No. 62-22945 and so on. With the configuration of this curled portion, it is possible to prevent the external water from entering into spaces of the curled portion and adhering to the trim end of the curled portion. Therefore, it is effective in view of rust resistance of the trim end of the curled portion of the steel can.
In case of the aluminum can, there is no possibility that the commercial value is deteriorated due to the rust on the trim end of the curled portion, unlike the steel can. However, regardless of whether the can is made of aluminum or steel, when the end portion of the opening side of the can body made of the resin coated metal sheet is trimmed in the manufacturing process, the resin film at the trim end may partially get fluffed like strings and peeled. If the curled portion formed on the opening portion is curled outwardly in this case, the fluffed and peeled resin film can be seen from outside at the lower end of the curled portion, thereby deteriorating an appearance of the can. Accordingly, it is preferable to form the outwardly curled portion by curling the trim end inside of the curled portion.
However, the known configuration of the curled portion thus formed, e.g., the configuration disclosed in Japanese Published Examined Application No. 56-14051, Japanese Utility Model Laid-Open No. 56-24431 (ref. FIG. 6), does not seem to have sufficient deformation resistance in view of strength. The curled portion may be deformed when it receives the great pressure from above or experiences the drop impact. As a result of this, the sealability between the curled portion and the cap (or a seal member) may be deteriorated.
On the other hand, according to the configuration as disclosed in, e.g., Japanese Utility Model Laid-Open No. 61-51314 (ref. FIG. 6) and Japanese Utility Model Laid-Open No. 62-22945, an upper end of the neck portion is curled doubly, after a threaded cylindrical body made of resin is fit into the neck portion, therefore, it seems excellent in strength. However, if the curled portion is not constantly pressed hard against the resin cylindrical body, the curled portion may slack due to springback, and as a result, the curled portion is unwound little bit. Accordingly, the springback varies a shape or dimensions of the curled portion, and this may cause a variation in the sealability between the curled portion and the cap (or a seal member).
In order to solve the fluctuation of the sealability between the curled portion and the sealing member, it is conceivable that the threaded cylindrical body made of resin is inserted into the neck portion and fixed by the curled portion. However, the threaded cylindrical body made of resin cannot be easily removed from the metal can body (i.e., the bottle-shaped can etc.), and not so small to be neglected at the stage of recycling. Therefore, a recycling efficiency of the can body is degraded.
The first object of the present invention is to provide a curl configuration of the opening portion of the metal can, which can solve the above-mentioned problems. Specifically, the object of the present invention is to provide a curled portion which is formed such that the trim end of the curled portion is rolled in and confined at the opening portion of the can. With this curled portion according to this invention, the trim end is prevented from gathering rust; the hair-like string resin of film at the trim end is hidden; and the adequate sealability is attained between the curled portion and the cap (i.e., a seal member), without neither being deformed by the pressure from above or drop impact, nor causing variation in its shape and size due to the springback.
The second object of the present invention is to provide a method for forming the curled portion having such a configuration.
To form the outwardly curled portion on the opening portion of the can, conventionally, a flanging is applied to the opening portion of the can, the portion around which is still in a cylindrical shape, by e.g., pressing from above by a disc-shaped head having at its lower end a working face in a circumferential direction, or by contacting a plurality of working rollers held rotatably by a rolling head with the opening portion and thereby pressing from above with rotating the rolling head. Then, the flanged portion is so curled as to inflect downwardly, thereby forming the curled portion which has a generally round cross-section.
On the contrary, the outwardly curled portion of the metal can according to the present invention is formed differently from the conventional curled portion having a generally round cross-section, as will be described hereinafter. As illustrated in FIGS. 5 and 11, first of all, the trim end of the opening portion is folded outwardly over a predetermined length, and then this folded portion is flanged outwardly and further folded downwardly. As illustrated in FIG. 5, such flanging and folding are repeated again. As compared to the conventional forming of the curled portion (i.e., flanging and curling), a great power is required for such flanging and folding to form the curled portion.
Specifically, in the forming process of the curled portion, a metallic material of the processed portion is elongated at the stage of flanging, because the processed portion moves outwardly so that the diameter increases. On the contrary, the metallic material of the processed portion is contracted at the stage of folding, because the processed portion moved outwardly is then displaced inward and downward so that the diameter decreases. Therefore, the processed portion as thus elongated and contracted is simply a single configuration according to the curled portion of the prior art having the generally round cross-section, whereas according to the curled portion of the present invention, the processed portion is a multiple configuration (i.e., a two-fold configuration after the first flanging and folding, and a three-fold configuration after the second flanging and folding). It follows that the greater force is required to elongate and contract the metal material of the processed portion as compared to the case of forming the curled portion according to the prior art; however, applying the great force (i.e., the pressure by a forming tool) at a working time may cause a damage on the resin film covering an inner face of the opening portion contacting with the forming tool.
The second object of the present invention is to solve the above-mentioned problem. Specifically, the present invention is aimed at providing a forming method of the curled portion of the metal can, in which the resin film covering the inner face of the opening portion of the can contacting the forming tool is not damaged, when forming the outwardly curled portion by folding the opening portion outwardly into two folds over the predetermined length from the trim end, and then flanging and folding so as to curl the trim end.